A striking syntactic feature of Rukai is that it has no focus system, as found in all other Formosan and western Austronesian languages. Focus system is generally believed to be an important and typical syntactic feature in all these Austronesian languages. However, there is no evidence that Rukai has had any focus system throughout its history.

Lexicostatistic evidence indicates that Paiwan is more closely related to Puyuma. However, Puyuma has a focus system different from all the other Formosan languages, including Paiwan, which has a typical Philippine type of focus system.